Electrolysis is a method widely used for the elimination of organic and inorganic pollutants in industrial effluents. From the point of view of electrochemical treatment of wastewaters, the processes of cathodic recovery of heavy metals, in use in many industries, is well known, though it is not mentioned explicitly herein because it is outside the scope of this invention.
The oxidation of waste substances by means of oxygen can be carried out directly in the electrolytic cell, as described in patent FR 227021 1, which uses the oxygen given off at the anode to produce a fast "biodegradation" of the organic compounds in septic tanks, with formation of a floating foam which is then removed.
Japanese patent JP 04197489 proposes treatment of the wastewater with a gas containing oxygen at approximately pH 10 and subsequent electrolysis of the treated water.
Patent JP 51019363 describes an electrolytic oxidation process in which different sugared compounds and lignin are totally oxidized to carbon dioxide and water on an anode, preferably of lead dioxide. In Swedish patent SU 962212, the wastewater is treated with a gas containing oxygen in a layer of granulated conductive material (for example, Ti and SiC) situated between two electrodes.
Patent SU 3200453 claims 98% purification of a vat-dyeing wastewater using a lead dioxide anode. In other cases, such a patent EP 231100, SU 966028 or JP 04118091, the current applied during the electrolytic treatment polarizes the pollutants and generates small gas bubbles which cause said residues to cluster together in lumps and rise to the surface, from which they are removed. This technique is termed electrofloating.
The electrochemical destruction of toxic organic pollutants in wastewaters has caused great interest in recent years. In particular, the studies undertaken on electro-oxidation of phenols [references 11-14 of the bibliography listed on pages 5 and 6] and anilines [14-19] in aqueous medium at different pHs has shown the formation of a wide variety of products (dimers, polymers, benzoquinone, maleic acid, etc.), depending on the electrolysis operating conditions and the chosen reaction medium. In all this works, special stress has been laid on the type of anode on which the electrochemical oxidation is carried out, reaching the conclusion that the best anodes are metals with high oxygen overtension. Platinum has undoubtedly been the anode most widely used in laboratory studies, due to its being a noble metal. Given the high price of platinum, however, alternative anodes have been sought, such as PbO.sub.2 and doped SnO.sub.2. A recent study [13] on the electro-oxidation of phenol has revealed that under certain conditions doped SnO.sub.2 is a more effective anode than PbO.sub.2, while the latter is actually better than platinum itself. Other types of electrodes, such as DSA.RTM. (Ti base), currently under full commercial development due to its great chemical inertia, are still not shown in the literature as anodes in the electro-oxidation processes of toxic pollutants such as phenols and anilines.
Rarely has account been taken of the effect of the cathode on the electro-oxidation of organic compounds, solid platinum or platinum deposited on a cheaper substratum such as titanium being used normally for this purpose.
Despite the work undertaken in the last few years on the electrochemical destruction of toxic organic pollutants in wastewaters, the bibliography contains no work in which their total elimination from the reaction medium has been achieved. In general, the use of anodes of Pt, PbO.sub.2, or SnO.sub.2 and a platinum cathode has achieved complete destruction of the initial organic compounds (anilines and phenols, for example [11-19]), though complete elimination of their intermediate reaction products has not been achieved.
In accordance with the bibliography [1-3], oxygen in an aqueous medium is reduced on a cathode of gold, mercury or graphite to generate hydrogen peroxide according to the electrochemical reaction: EQU O.sub.2 +H.sub.2 O+2e.sup.-.fwdarw.HO.sub.2.sup.- +HO.sup.- (1)
The HO.sup.2- ion, conjugate base of the hydrogen peroxide, is a good oxidant and can react with intermediate products of the oxidation process of organic pollutants, boosting their complete degradation into water, carbon dioxide and other inorganic compounds of toxicity levels tolerable to microorganisms (NH.sub.3, HCl, etc.). The efficiency of the hydrogen peroxide is especially marked if it is subjected to UV radiation of about 254 nm, which in addition to its disinfectant effect, can decompose photolytically the hydrogen peroxide generated in reaction 1 into hydroxyl radicals (after fluorine, the most oxidizing species known), much more reactive than the H.sub.2 O.sub.2 itself [10]. Another by no means negligible way of boosting the oxidizing effect of this compound is the presence of Fe(II) as catalyst at a pH close to 3, which also produces hydroxyl radicals, according to the Fenton reaction. Silver or cobalt ions can also be used.
Although reaction 1 can be carried out on various cathodes, we have found that oxygen diffusion cathodes are the ones best suited for the present process.
Reaction 1 is faster and can be controlled better on oxygen diffusion electrodes than on a simple graphite electrode. Oxygen diffusion electrodes may be made of a mixture of carbon (very fine particle lampblack) and a water-repelling polymeric agglomerant (preferably polytetrafluoroethylene, PTFE), pressed at about 350-400.degree. C. (pasty melting temperature of PTFE) on a metallic mesh generally of Ni, Ag or stainless steel) which acts as a current distributor [3-5]. The mission of the PTFE is to keep the carbon compact, with sufficient porosity to diffuse the oxygen gas and lend the whole a water-repellant character. Carbon-PTFE electrodes have been developed as components of fuel cells and some have been marketed as components of metal-air cells. Various patents have been filed over the last fifteen years describing different carbon-PTFE electrodes [2-4, 7] with applications as diverse as acting as cathodes in zinc-air cells [4-7] or being used as cathodes in electrolytic cells for the generation of base solutions of hydrogen peroxide [2,3].
Carbon-PTFE cathodes in alkaline medium are sensitive to the partial pressure of oxygen of the gas acting upon it, it having been found that the reaction speed (1) increases as the partial pressure of oxygen increases [8,9], so that circulation through them of current densities of up to 2 A/cm.sup.2 can be achieved when operating with oxygen pressures of up to 5 atmospheres.
U.S. Pat. No. 4,619,745, to Porta et al. describes a process for the electrochemical decontamination of polluted water that uses a cell containing a porous cathode, in the presence of oxygen. In this process, oxygen is generated in the anode through water electrolysis according to the reaction: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H+4e.sup.-.
The oxygen and oxygen dissolved in the polluted water is adsorbed on the carbon of the cathode and finally it is reduced, by applying a voltage, to obtain HO.sub.2.sup.-.
The cathode is a structurally porous carbon or graphite cathode, freely permeable to water, which provides a bed of adsorbent particles. The water with dissolved oxygen runs through the mass of the cathode, where the oxygen is reduced according to the reaction: EQU O.sub.2 +H.sub.2 O+2e.sup.-.fwdarw.HO.sub.2.sup..multidot. +OH.sup.-.
The oxidizing species HO.sub.2.sup..multidot., which is very active but unstable, is used as depolluting agent; HO.sub.2.sup..multidot. is decomposed to form active (nascent) oxygen, and the impurities present in the water are substantially treated by the presence of this active (nascent) oxygen.
When the degree of pollution is very high, an additional inlet is used, through which oxygen from an external source is bubbled or supplied into the cathode through which the water is flowing. This is additional oxygen that enters the cell with the same purpose as that added to the water before entering the apparatus.
The process of U.S. Pat. No. 4,619,745 has the following drawbacks:
First, the oxygen that dissolves in the water, or in the form of bubbles, imposes a limitation, since the current densities that can be used are limited by the solubility of oxygen in water. As a result, the oxygen contents reacting in the cathode is low, and the reaction products do not have a fast oxidizing effect on the pollutants due to their low concentration. Additionally, the oxygen bubbles within the cathode mass reduce the conductivity.
The cathode acts as a filter, with all the water flowing therethrough, so that impurities from water remain retained inside the carbon or graphite thereof, so that after a period of operation the process must be stopped and the cathode has to be cleaned from impurities.